Pressure exchangers are sometimes called “flow-work exchangers” or “isobaric devices” and are machines for exchanging pressure energy from a relatively high-pressure flowing fluid system to a relatively low-pressure flowing fluid system. The term fluid as used herein includes gases, liquids, and pumpable mixtures of liquids and solids.
In some industrial processes, elevated pressures are required in certain parts of the operation to achieve the desired results, following which the pressurized fluid is depressurized. In other processes, some fluids used in the process are available at high-pressures and others at low-pressures, and it is desirable to exchange pressure energy between these two fluids. As a result, in some applications, great improvement in economy can be realized if pressure can be efficiently transferred between two fluids.
By way of example, there are industrial processes where a catalyst is utilized at high-pressure to cause a chemical reaction in a fluid to take place and, once the reaction has taken place, the fluid is no longer required to be at high-pressure, rather a fresh supply of fluid is required at high-pressure. In such a process, a pressure exchanger machine can be utilized to transfer the pressure of the reacted high-pressure fluid to the fresh supply of lower pressure fluid, thus improving the economy of the process by requiring less pumping energy be supplied.
Another example where a pressure exchange machine finds application is in the purification of saline solution using the reverse osmosis membrane process. In this process, an input saline solution stream is continuously pumped to high-pressure and provided to a membrane array. The input saline solution stream is continuously divided by the membrane array into a super saline solution (brine) stream, which is still at relatively high-pressure, and a purified water stream at relatively low-pressure. While the high-pressure brine stream is generally no longer useful in this process as a fluid, the flow-pressure energy that it contains has a high value. A pressure exchange machine is employed to recover the flow-pressure energy in the brine stream and transfer it to an input saline solution stream. After transfer of the pressure energy from the brine stream, the brine is expelled at low-pressure to drain by the low-pressure input saline solution stream. Thus, the use of the pressure exchanger machine reduces the amount of pumping energy required to pressurize the input saline solution stream.
U.S. Pat. Nos. 4,887,942 and 6,537,035 disclose a pressure exchanger machine for transfer of pressure energy from a liquid flow of one liquid system to a liquid flow of another liquid system. This pressure exchanger machine comprises a housing with an inlet and outlet duct for each liquid flow, and a cylindrical rotor arranged in the housing and adapted to rotate about its longitudinal axis. The cylindrical rotor is provided with a number of passages or bores extending parallel to the longitudinal axis and having an opening at each end. A piston or free piston may be inserted into each bore for separation of the liquid systems. The cylindrical rotor may be driven by a rotating shaft or by forces imparted by fluid flow. Since multiple passages or bores are aligned with the inlet and outlet ducts of both liquid systems at all times the flow in both liquid systems is essentially continuous and smooth. High rotational and thus high cyclic speed of the machine can be achieved, due to the nature of the device, with a single rotating moving part, which in turn inversely reduces the volume of the passages or bores in the rotor, resulting in a compact and economical machine.
U.S. Pat. Nos. 3,489,159, 5,306,428, 5,797,429 and PCT Patent Publication WO 2004/111509 all describe an alternative arrangement for a pressure exchanger machine, which utilizes one or more fixed exchanger vessels, with various valve arrangements at each end of such vessels. These machines have the advantage of there being no clear limit to scaling up in size and, with the device of WO 2004/111509, leakage between the high-pressure and low-pressure streams can be minimized. A piston may be inserted into each exchanger vessel for separation of the liquid systems.
Disadvantages of pressure exchange machines based upon U.S. Pat. No. 4,887,942 can include: that for high flow rates it is necessary to increase the size of the cylindrical rotor, and there are limitations on the amount that such a rotor can be scaled up as the centrifugal forces will attempt to break apart the rotor, similar to the problems encountered in scaling up flywheels to large sizes and speeds; that very small clearances are required between the cylindrical rotor ends and the inlet and outlet ducts to maintain low rates of leakage between the high-pressure and low-pressure fluid systems, with such leakage causing a reduction in efficiency and it being difficult to maintain such small clearances; that when operated at relatively high rotational speeds, it may not be practical to utilize a driven shaft to control rotation of the rotor, rather by non-linear forces imparted by fluid flow which can reduce the flow range over which a given device can operate efficiently; and that when operated at relatively high rotational speeds, it may not be practical to utilize a piston in the passages in the rotor, thus reducing efficiency by increasing mixing between the two fluid streams.
Disadvantages of pressure exchange machines based upon U.S. Pat. No. 3,489,159 can include: that the flow in both fluid systems is not essentially continuous and smooth unless a large number of exchanger vessels are utilized; that these devices are generally limited to low cyclic speeds due to the linear or separated nature of the valves, thus requiring relatively large volume exchanger vessels, which increases cost and size; and that due to the multiple moving parts, these devices tend to be more complex and expensive to manufacture than devices based upon U.S. Pat. No. 4,887,942.